When writing my thesis, I was writing a part about SQL data types and how to choose them wisely when designing a database structure.
I read somewhere that as a best practice you should not assign negative values to PK. this lead to the following question:
will an instance of SQL Server assign negative values to a Primary Key by default ? I know it is possible to assign them yourself, but I was wondering if SQL server will assign them by default and if so, in which cases?
I assume, but I've never tried, that if you create a primary key with a designation of IDENTITY(0,-1), it will auto assign descending values starting from 0
There lies a difference between Primary Key and Identity.Identity Property creates an identity column in a table with syntax:
IDENTITY [ ( seed , increment ) ]
where increment is the incremental value that is added to the identity value of the previous row that was loaded which can be + or - depending on the requirement.
and Primary Key is a column or combination of columns that contain values that uniquely identify each row in the table. It can be any set of columns other than the Identity column if they garuntees unique identification of rows in the table.
Coming back to the question a table can have only one PRIMARY KEY constraint, and a column that participates in the PRIMARY KEY constraint cannot accept null values. Because PRIMARY KEY constraints guarantee unique data, they are frequently defined on an identity column.
So SQL server don't assign any default value to a primary key as it's a constraint. When you specify a PRIMARY KEY constraint for a table, the Database Engine enforces data uniqueness by creating a unique index for the primary key columns.
Related
One of the values to be inserted into a table is a foreign key. I need to figure out how to, inside the CHECK clause, check if a value in another table is equal to a specific value using the foreign key (which is unique in the other table).
Example Diagram
As an example, the MonkeySpecies table has a unique primary key. I need to make it so that the Monkey table can only be added to if the SpeciesID in the MonkeySpecies table is not 'extinct'.
Does defining the constraint PRIMARY KEY already makes sure that the column values are unique and not null or do you have to define it seperately?
Yes. But one exception is Sqlite.
See https://sqlite.org/lang_createtable.html
Each row in a table with a primary key must have a unique combination of values in its primary key columns. For the purposes of determining the uniqueness of primary key values, NULL values are considered distinct from all other values, including other NULLs. If an INSERT or UPDATE statement attempts to modify the table content so that two or more rows have identical primary key values, that is a constraint violation.
According to the SQL standard, PRIMARY KEY should always imply NOT NULL. Unfortunately, due to a bug in some early versions, this is not the case in SQLite. Unless the column is an INTEGER PRIMARY KEY or the table is a WITHOUT ROWID table or the column is declared NOT NULL, SQLite allows NULL values in a PRIMARY KEY column. SQLite could be fixed to conform to the standard, but doing so might break legacy applications. Hence, it has been decided to merely document the fact that SQLite allows NULLs in most PRIMARY KEY columns.
can a unique column in SQL be auto_increment,...
i.e. if we have a database table, which has a Primary Key Column "id" with identity (1,1). Then, can we have any other column with UNIQUE constraint and IDENTITY(1,1) with it.?
You can only have one identity column per table, but you can have as many unique constraints (or indexes) as you want.
In fact, if you are using an int identity column as your primary key, it's highly recommended to have at least one more unique index on your table.
The reason for this is that using a surrogate primary key without enforcing uniqueness on the natural key means your database can't enforce data integrity correctly. (Bonus reading - Natural vs. Surrogate Keys in SQL Server : Getting the Proper Perspective)
BTW, the fact that the column is an identity column does not mean it's unique. It will only be unique as long as no one mess with it - identity columns can be inserted explicit values using set identity_insert on, and can be re-seeded using DBCC CHECKIDENT sql command.
The answer is no.
Only one identity column can be created per table.
Read reference here: https://learn.microsoft.com/
I have a table with 2 primary key columns : ID and StudentID.
ID column is set to isIdentity = Yes with auto increment.
I've tested it multiple times before, but for some reason this time, when I insert a duplicate value on StudentID, it does not throw the error but instead added it on to the database. 2 of the same values are displayed when I show the table data.
What can be the problem here?
You have a compound primary key on ID and StudentID. That means you the combination of ID and StudentID together must be unique. Since ID is an identity column that combination of ID and StudentID will always be unique (because ID is already unique on its own).
You can change the primary key to be on ID only. Then you can add a unique index on StudentID. For example:
create unique index idx_studentID on yourTable(StudentID)
That will insure that the StudentID column, in fact, contains only unique values.
It seems like you may not actually need ID column, but that's a little wider discussion than your original question.
You can't have 2 "primary keys". You can have a compound primary key (meaning the combination needs to be unique, which is what it sounds like you have now. Or, You can have one "primary" key and one "unique" constraint which is what it sounds like you want.
You cannot have 2 Primary Keys. You can have multiple Unique Keys if needed, which should help you in your case. Make sure to go back to your table creation and double check which column is your Primary Key and work from there.
Do not mix up identity, primary key and unique key.
Any table can have identity key which you can setup on table. Here seed can be say 1, then increment it by 1. So incremental order will like 1,2,3...and so on.
Primary key, one can define on specific column of the table. Identity key can be used as primary key. But you can have identity column as well primary key on same table. Primary key is one and only for the table.So if you are treating identity as primary key, then you will have no further table column as primary key.
Unique key, can be more than one column with your table.
While fetching rows from table data, if you provide combination of identity key, primary key and unique key then search will be fastest
During my first response, I have mentioned that one can generate identity column by soft coding and it will not be treated as primary key.Following is syntax one can use while creating table.
1] If one wish to set identity column as primary key
--id int identity(1,1) primary key
2] If one doesn't wish to set identity column as primary key and still wish
to us identity column then donot us word primary key for identity column.
--id int identity(1,1)
In this 2] case scenario, one may create primary key on other table column.
I have to insert data into a table that has a PK in it. I also have another table that has a clustered index in it.
Should I drop the PK or the INDEX for the the best INSERT speeds? Then recreate them afterwards?
I load data to these types of tables on a routine basis and I want to make sure I am using the quickest way possible in all situations.
A primary key uniquely identifies a record and has other uses as well. An index makes select queries run faster.
You should never drop your primary key.
Whether or not you drop and re-create indexes when adding records depends on the circumstances.
Primary Key : Uniquely identifies the the data & we cannot insert duplicate Data.
Index : Index help us to get out data to us very quickly.
Very Important Concept about Primary key & Index
Suppose your column is marked with the primary key then Clustered Index automatically gets created,
If no clutstered index is already present in the table
To See that Your Index is Created Successfully, You can use.
sp_helpindex Index_Name
- About Index :
You cannot create a unique index on a single column if that column contains NULL in more than one row. Similarly, you cannot create a unique index on multiple columns if the combination of columns contains NULL in more than one row. These are treated as duplicate values for indexing purposes.
- About Primary Key :
All columns defined within a PRIMARY KEY constraint must be defined as NOT NULL. If nullability is not specified, all columns participating in a PRIMARY KEY constraint have their nullability set to NOT NULL.